There are different possible methods for applying an embossing pattern to tissue paper. The most common are point to point, nested, structural embossing or double embossing random laminate. In each of these cases, the embossing pattern is applied by use of one or more steel rolls in combination with rubber rolls. The rubber rolls have a yielding surface. A raised embossing pattern on the steel roll elastically presses into the rubber roll so that the tissue paper fed between the rubber roll and the steel roll is imparted an embossing pattern. All plies of a multi-ply tissue paper are thus embossed and are normally glued together. Alternatively, it is possible to provide a mechanical ply-bonding by edge-embossing both plies of tissue paper together.
In order to meet customer preferences, many multi-ply tissue paper products are provided with a background embossing pattern and a superimposed spot (motive) embossing pattern. The background pattern which will be denoted microembossing pattern in the following, normally consists of a regular structure of small protuberances in the tissue ply which is formed by small protrusions on an embossing roll which press against and into the tissue ply to be embossed. The spot embossing pattern, on the other hand, mainly consists of larger decor elements which are formed by a superposition over the microembossing pattern.
It is possible to provide embossing rolls in which both the microembossing pattern and the spot embossing pattern are engraved on the circumferential surface. However, if it is desired to change the spot embossing pattern, the whole embossing roll has to be exchanged resulting in rather long changeover times to exchange the rolls having a weight up to several tons. To exchange such a full size embossing roll, the changeover time can be in the range of several working days.
To overcome this problem, it has been proposed to use a separate spot embossing unit for imparting a graphic pattern to a tissue web already provided with a microembossing pattern. Such an additional unit consists of a spot roller having a graphic pattern on its circumferential surface and cooperating with a rubber roller. The additional spot roller has to be driven so that an additional driving unit and drive control are necessary.
FIG. 3 shows a conventional device, as part of a paper converting machine, which has a separate spot embossing device 100 consisting of a steel engraved spot embossing roll 110 and a rubber anvil roll 120. The product 12 already ply-bonded together in a nested configuration passes through this spot embossing device 100 and leaves it as product 12′ with an additional spot (motive) embossing.
The prior art device shown in FIG. 3 consists of two microembossing stations which impart a background embossing pattern to the two plies 10, 11 of tissue paper. Each microembossing unit consists of a steel roll 14a, 14b which cooperates with a rubber roll 16a, 16b. Between each steel embossing roll and the corresponding rubber roll, a nip is formed through which the plies of tissue paper pass. The steel embossing rolls 14a, 14b have protrusions 18a, 18b on their circumferential surface whose height, for an easy understanding of FIG. 3, is not drawn to scale but highly exaggerated. The protrusions 18a, 18b press into the yielding circumferential surface of the corresponding rubber roll 16a, 16b so that the ply of tissue paper running through the nip receives an embossing pattern with small protuberances.
In the example shown in FIG. 3, the two steel embossing rolls 14a, 14b are positioned relative to each other so that the two plies of tissue paper 10, 11 are brought together in a nested configuration. Additionally, the ply 11 of tissue paper receives an adhesive applied by use of the adhesive rolls 22 to that surface which comes into contact with the other ply 10 of tissue paper. After having brought together the two plies 10, 11 in a nested configuration, the combined two ply tissue product 12 passes through the nip between the steel embossing roll 14b and a marrying roll 20 in which the two plies are sufficiently pressed together so that the two plies are bonded together. Then, the product 12 leaves this part of the paper converting machine and is passed on to the aforementioned spot embossing device 100 which is operated as a separate unit of the paper converting machine.